I've had mine for years. Waterproof as hell, comfortable and durable.
Personally, I'd go with the Large bag myself. At first, it felt WAY too big. But the material softens and flattens against your body and it gets very comfortable. I've got a medium shoulder bag (used for walking trips/vacationing), a small (great work briefcase) and a Large for the big commutes.
The folks who run it are very responsive and friendly and they go out of their way to be cool. Case in point, I found the strap on my L bag to be a little stiff. So, I sent it in for a softer strap. He noticed that I wore mine low, so he added an extra foot of strap for free. Had no idea he was going to do it, but when I got it back, I was blown away by their thoughtfulness. They just go the extra mile.
Bottom line, I think Courierware is the best. If you go through them you won't be disappointed. let me know if you have any other questions.